Freezing potentials in heavy water

Freezing Potentials in Heavy Water W. D R O S T - H A N S E N ~ND R I C H A R D W. C U R R Y

Institute of Marine and Atmospheric Sciences, University of Miami, Miami, Florida 33149 Received December 6, 1969; accepted December 13, 1969 The freezing of a dilute aqueous solution is accompanied by a charge separation (WorkmamReynold's "freezing potential") between the advancing ice front (negatively charged) and the unfrozen solution (positively charged). Potentials of 10 and 25 V are observed when freezing 10-3 and 10-4 M KC1 solutions at a rate of 20 t~/sec. The potential is generated within seconds after freezing has begun, reaches a maximum value, and decays slowly as freezing progresses. Gentle stirring obliterates the potential. The freezing of 10-3 and 10-~ M KC1 solutions in heavy water (D~O) has now been studied. The observed behavior is similar to that observed in H~O. Freezing potentials occur in the solidification of dilute solutions of electrolytes in D~O as with ordinary water. The magnitude of the freezing potentials observed with heavy water is similar to that observed with ordinary water (H~O), both with respect to magnitude and concentration dependence. Notable differences are observed only for very low concentrations where a reversal of sign of the potential is observed.

I. INTRODUCTION Workman and Reynolds (I) discovered the existence of a potential between a growing ice crystal and the unfrozen solution. Many studies have been made of such freezing potentials in dilute aqueous electrolyre solutions (in ordinary water, H20), but there appear to be no reports on similar potentials during the freezing of electrolytes in heavy water (D20). The present study was undertaken in order to: (a) determine if potentials occur in the freezing of dilute solutions of heavy water and (b) determine if such potentials exhibit unexpected behavior as the result of the suspected higher degree of structuring in heavy water compared to ordinary water (probably reflecting a stronger hydrogen (deuterium) bond in D20). Not unexpectedly, the over-all freezing potentials observed with solutions in heavy water rather closely parallel those found in ordinary aqueous solutions; however, in the more dilute solutions, the results are not identical. II. MATERIALS AND METHODS The apparatus used in this experiment was essentially the same as that used by

W o r k m a n and Reynolds (1). The apparatus consists of a large heat sink, freezing cup, electrometer, and recorded. The heat sink was constructed in the following manner: a large copper rod (4 inches diam. × 4.5 feet long) was bolted to an aluminium sled (see Fig. 1). This assembly was placed in a commercially available deep freezer. To increase the thermal capacity, the deep freezer was partially filled with an ethylene glycol-water mixture (60% v/v). A 0.5 inch plexiglass sheet was used as a cover for the deep freezer. The copper rod was allowed to protrude through the cover b y about 9 inches. The plastic cover and protruding copper bar were insulated with Dow-Corning styrofoam F R insulation (1.5 inch thick). A 20 W flexible heater (Watlow Electric Manufacturing Company) (4 inches wide X 12 inches long X 1/~2 inch thick) was wrapped around the copper rod. This heater was connected to a Y S I temperature controller, the sensing probe of which was located on the axis of the copper rod approximately 3/~ inch below the upper surface. The temperature controller is capable of maintaining the copper rod at constant tern-

Journal of Colloid and Interface Science, ¥ol. 32, No. 3, ~ a r c h 1970

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will possess a lattice strain owing to the crystallographic misfit of neighboring molecules caused by the need to open the bond angle (by about 2-4°). In the absence of ionic impurities, the ice lattice adjusts to this lattice strain by developing a microstructure. If suitably sized ions (anions) are present in solution, these may selectively become incorporated into the ice and relieve the lattice strain by a slight (crystallographic) rearrangement of the water molecules. This results in the generation of the potentials and the elimination of the microstructure. The mechanism for the incorporation of the ions is seen in terms of the "adsorption" of the anions onto an oriented, polar layer of ice at the water/ice interface, caused by alignment of the water dipoles. Polar types of ice are well known among the high-pressure polymorphs: Ice II and Ice VIII are both polar. Attention is called to the fact that although Ice II and Ice VIII are thermodynamically unstable near 0°C and 1 atm, the energy difference between Ice I and Ice II, for example, is only 19 cM/g-mol (compared, for instance, to the energy difference between the liquid and crystal (Ice I) of 1440 cal/gmol). Likewise, the entropy difference is relatively small--0.8 eu. Thus, although basically unstable thermodynamically, Ice II (or possibly Ice VIII) might well be "ldnetically favored" in the process of solidification. Before proceeding, it should be noted that between the advancing ice front (over-all hexagonal structure) and the short-range ordered (but unknown) bulk water structure,

III. RESULTS AND DISCUSSION

The senior author of this paper has discussed the origin of the freezing potentials in a previous paper (2). The essential elements which have been postulated to explain the phenomenon are the following: the bond angle HOH in liquid water--and possibly in ice--is probably about 105 ° (to 108°); i.e., different from the perfect tetrahedral angle (109°28'). As a result, a single crystal of ice

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a transition zone may exist of more or less complete disorder. This transition zone consists probably primarily of water monomers. The reason for postulating the existence of a disordered region is the fact that the bulk water structure may very likely prove not to be Ice Ih-like. Thus, there may, of necessity, be a "lattice misfit" between the advancing ice front and the bulk water structure. Naturally, in this connection, the phrase lattice misfit should be taken in the loosest possible sense, as a rigid lattice may not be involved in the freshly formed ice, and certainly not in the liquid water. However, granted that the structures are likely to be considerably different, it is then reasonable to expect that the transition zone is characterized by the enhanced disorder necessary for the process of transferring the individual water molecules from whatever the main structures of bulk water may be to the hexagonal structures of the ice lattice. It is in connection with this dynamic process that it is conceivable that a polar form of ice may well be the "kinetically favored" form for the ice deposited. We now address ourselves to the results obtained in the present studies on electrolytes in heavy water, compared to ordinary water. X-Ray crystallographic studies have strongly suggested identical structures for solid H20 and solid D20. However, recall that the melting points differ by approximately 3.82°C and the heats of melting by about 65 cal/g-mol. These differences have I

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been attributed to different degrees of structuring in the liquid and particularly to different hydrogen (deuterium) bond energies. (For a discussion, see the monograph by Eisenberg and Kauzmann (3) or the recent review article by Barclay Kamb (4).) We note in passing that the proposed mechanism for the generation of the freezing potentials involves the existence of a transient polar ice at the ice/water interface. Whereas the polar structure may very likely be Ice II, this point is certainly not proven, and any other local, polar structural arrangements of the water molecules might satisfy the requirements for selective ion adsorption. However, it is to be expected that if transient, polar structures occur, they will likely have densities different from that of ordinary ice. In this connection, it is of interest to note that Dantl and Gregora (5) recently reported density differences between freshly frozen ice and ice aged for periods of hours. The difference in density was approximately 0.003 gm/cm 3, the newly formed ice being more dense than the aged samples. This would be consistent with a transitional structure such as Ice II, which has a density of approximately 1.18 gm/cm 3. Figure 3 shows the observed freezing potentials, measured as described in the section on "Experimental Procedure." Three different aspects of these results will now be discussed. First, it is observed that the maximum potentials are probably identical (within the experimental error). Considering I

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FREEZING POTENTIALS IN HEAVY WATER the rather small difference between ordinary water and heavy water for various properties (for instance, dipole moment, dielectric constant, and viscosity), it is probably reasonable to expect the freezing potentials for the two systems to be rather similar. Thus, notwithstanding the different melting points, it is reasonable to assume that the energeties of the lattices relative to the respective liquid phases are more or less similar. Second, it is observed that the concentration resulting in maximum freezing potential for ordinary water is larger than the corresponding concentration in D20. It is not certain if this difference is real, although we presently tend to believe that this difference is significant. Because of the uncertainty, we do not wish at this time to elaborate further on the difference in these concentrations. Third, we note the reversal in sign of the freezing potential during the freezing of highly dilute solutions in heavy water. This difference appears to be real. It is interesting to note that the reversal in sign of the freezing potentials with heavy water occurred approximately at the concentration where the freezing potential with ordinary water has essentially decreased to zero. The reversal in the sign of the freezing potential may be highly significant. Before proceeding, attention is called to the fact that Parreira and Eydt (6) and, more recently, Murphy (7) have observed similar reversals in sign for the freezing of ordinary water. Such a reversal has not been observed by the present authors during the freezing of solutions in ordinary water. According to Gross (8), it now seems likely that the results obtained by Parreira and Eydt are in error. It is probable that the reverse potentials observed by these authors were merely the potentiMs one should expect upon freezing very dilute solutions of ammonia. It is well known that ammonia in extremely low concentrations (say, 10.8 M) may easily occur as an accidental contamination, possibly from the laboratory air. However, elsewhere the senior author of this paper has speeulated upon the possibility that reverse potentials may, indeed, exist. These potentials would result from the dipolar fields created at the ice/water interface (compare also Parreira

467

and Eydt). It is, indeed, these dipolar fields which cause the selective adsorption of the ions, which gives rise, in turn, to the charge separation. However, the dipolar fields should not be able to sustain a current and this has been confirmed experimentally in the present study. The reverse potentials obtained for heavy water (for a concentration below 4 × 10.4 M) decreased essentially to zero upon "shorting" the external circuit with a 1000-megohm resistor. Unfortunately, very few conductivity studies have yet been made on ice frozen from heavy water. Thus, the influence on the generation of freezing potentials of ice resistivity and possible effects of differences in the ice/metal electrode interfacial resistance (between ordinary ice and heavy ice) are not known. A significant difference in the yields of charge transfer across the ice/metal interface might possibly influence the potentials observed. Recall in this connection the large difference in proton versus deuteron mobility (0.08 compared to 0.02 em 2 V-1 see -1 for ordinary ice and heavy ice, respectively) and the different rate constants for the dissociation and recombination of H20 (D20). IV. SUMMARY Freezing potentials are generated during the solidification of dilute electrolytes in heavy water, as well as in ordinary water. The maximum potential and the coneentration at which the maximum potential is observed are approximately the same for solution in ordinary, as in heavy, water. Thus, it is reasonable to infer that the underlying mechanism is identical in the two cases. The higher deuterium bond strength in heavy water does not seem to play a significant role in the phenomenon. For extremely low concentrations of electrolytes, potentials occur which have the opposite sense as those encountered for higher concentrations (and in all cases studied with ordinary water used as solvent). No definite explanation can be offered for the reversal in sign at this time; however, the phenomenon could conceivably be related to differences in the dissociation/ assoieation process and/or proton (deutron) mobility in the solid phase. Another interesting difference between the freezing potenJournal of Colloid and Interface ,Science, Vol. 32, No. 3, March 1970

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rials observed for solutions in heavy water compared to ordinary water is the effect of mechanical stirring. As reported previously b y various authors (1, 2), stirring of ordinary aqueous solutions (H20) during freezing eliminates the potential almost instantaneously. However, stirring of solutions in D20 during freezing often enhances the potentials significantly. ACKNOWLEDGMENT The authors are grateful to the U. S. Department of the Interior, Office of Saline Water, for its support, which made this study possible.

Jourr~l of Colloid and Interface Sciew~e,eel. 32, No. 3, Msrch 1970

REFERENCES 1. WORKMAN,E. J., AND REYNOLDS, S. E., Phys. Rev. 78,254 (1950). 2. DROST-HANSEN,W., J. Colloid Interfac. Sci. 25, 131 (1967). 3. EISENBERG, D., AND KAUZMANN, W., "The Structure and Properties of Water." Oxford University Press, New York, 1969. 4. K~MB, B., J. Chem. Phys. 43, 3917 (1965). 5. DANTL, G., AND GI~EGORA,I., Naturwiss. 55(4), 176 (1968). 6. PARREIRA, H. C., AND EYDT, A. J., N a t u r e 208(5005), 33 (1965). 7. MvaP~v, G., Personal communication. (1969). 8. GROSS,G. W., Personal communication. (1969).